Near infrared spectroscopy (NIRS) is a non-invasive, optical technique for monitoring tissue oxygenation. NIRS relies on the relative transparency of tissues to near infrared light (700-900 nm) where oxygenated- and deoxygenated hemoglobin and cytochiome aa3 have distinct absorption spectra. Depending on the instrument, NIRS systems monitor oxygenation as hemoglobin-O2 saturation (SO2), cytoclirome aa3 redox state, or oxy- and deoxy-hemoglobin concentrations. NIRS differs from other oxygenation monitors such as pulse-oxrmetry in that it monitors parenchymal and microcirculatory (eg, capillaries) oxygenation to reflect tissue oxygen supply relative to demand. In clinical medicine, NIRS has been used as a research device to follow cerebral oxygenation during surgery and critical illnesses.
Despite its applicability and availability for several years, NIRS has not been widely utilized in clinical medicine. Uncertainties concerning optical pathlength and light scattering within the tissue have precluded absolute quantitation, limiting NIRS systems to describing relative oxygenation over time. The lack of a standard measure for NIRS has complicated assessment of its accuracy. Because NIRS monitors a tissue field containing capillaries, arteries, and veins, its calculated SO2 represents a mixed vascular SO2. No other method exists at present to measure this mixed vascular SO2. Before NIRS can be evaluated in clinical trials, essential for widespread use, absolute quantitation is required. Several approaches have recently been explored to improve NIRS quantitation. For example, application of radiative transport theory and time or frequency domain spectroscopy permits absolute quantitation through the determination of tissue absorption coefficients (xcexca), eliminating uncertainties in optical pathlength and light scattering. Although absolute quantitation of cerebral SO2 is theoretically possible with time-domain and frequency-domain NIRS (fdNIRS), their accuracy remains untested.
Accordingly, there is a need for NIRS instrument wherein absolute oxygenation levels can be determined quickly and accurately in a clinical setting.
The present invention comprises a method of determining the oxygenation level of tissue. The tissue is irradiated by a near infrared light source whereby the incident light passing through the tissue is detected by a light detector. Specifically, light signals of a single frequency at at least three separate wavelengths are provided from the near infrared light source. The near infrared light signals are collected with the light detector and, the phase differences between the collected near infrared light signals and a reference near infrared light signal are determined. The phase differences are used to calculate the oxygenation level of the tissue.
In another aspect of the present invention, a method of determining the oxygenation level of tissue comprises irradiating tissue with a near infrared light source, the incident light passing through the tissue to a light detector. Specifically, light signals of a single frequency at three separate wavelengths are provided from the near infrared light source. The near infrared light signals are collected with the light detector and the collected signals define a first, a second, and a third light signal having respective wavelengths xcex1, xcex2, and xcex3. The collected light signals are compared with a reference near infrared signal and the difference in phase between the first and third collected signals xcex8(xcex1xe2x88x92xcex3) is determined. The difference in phase between the second and third collected signals xcex8(xcex2xe2x88x92xcex3) is determined and a phase difference ratio of xcex8(xcex1xe2x88x92xcex3)/xcex8(xcex2xe2x88x92xcex3) is defined. The oxygenation level of the tissue is derived from the phase difference ratio and known tissue absorption constants.